The present invention relates to the area of industrial organic chemistry. More precisely, the present invention relates to a process for effectively removing formaldehyde from methylolalkanal solutions. The present invention furthermore relates to a process for hydrogenating methylolalkanals thus obtained to give polyols.
The condensation of formaldehyde with CH-acidic higher alkanals to give methylolalkanals, in general dimethylol- and trimethylolalkanals, and conversion of the compounds obtained to polyols constitute a widely used process in chemistry. Examples of important triols obtained in this manner are trimethylolpropane, trimethylolethane and trimethylolbutane, which are widely used for the production of finishes, urethanes and polyesters. Other important compounds are pentaerythritol, obtainable by condensation of formaldehyde and acetaldehyde, and neopentylglycol from isobutyraldehyde and formaldehyde. The tetrahydric alcohol pentaerythritol is also frequently used in the coatings industry but is also very important in the production of explosives.
The polyols can be prepared by various processes. One method is the Cannizzaro process, which is further subdivided into the inorganic and the organic Cannizzaro process. In the inorganic variant, an excess of formaldehyde is reacted with the corresponding alkanal in the presence of stoichiometric amounts of an inorganic base, such as NaOH or Ca(OH)2. The methylolalkanal formed in the first stage reacts in the second stage with the excess formaldehyde in a disproportionation reaction to give the corresponding polyol and the formate of the corresponding base, i.e. for example sodium formate or calcium formate. The production of these salts is a disadvantage since they are difficult to separate from the reaction product and moreover one equivalent of formaldehyde is lost.
In the organic Cannizzaro process, a tertiary alkylarnine is used instead of an inorganic base. Consequently, higher yields can be achieved than with an inorganic base. Trialkylanmonium formate is obtained as an undesirable byproduct. Thus, here too, one equivalent of formaldehyde is lost.
The disadvantages of the Cannizzaro process are avoided in the hydrogenation process. Here, formaldehyde is reacted with the corresponding alkanal in the presence of catalytic amounts of an amine. Consequently, the reaction stops at the methylolalkanal stage. After removal of the formaldehyde, the reaction mixture, which still contains small amounts of the corresponding triol in addition to the corresponding alkanal, is subjected to a hydrogenation in which the desired polyol is obtained.
Different variants of these hydrogenation processes are described, inter alia, in the applications DE-A-25 07 461, DE-A-27 02 582, DE-A-27 14 516, DE-A-28 13 201, DE-A-33 40 791 and WO 98/28253.
A major problem of the hydrogenation process is effective removal of the formaldehyde used in excess, after the condensation reaction, before the use of the reaction mixture in the hydrogenation. Effective removal of the formaldehyde is desirable simply because it can be recycled to the reaction and is thus not lost. In principle, it would be possible to hydrogenate the reaction mixture directly after the condensation without prior removal of the formaldehyde. However, this would be converted into the relatively useless methanol; furthermore, an increased consumption of hydrogen and a higher catalyst loading would also be the result, so that an unfavorable balance would be obtained in the end.
The literature describes various methods for effectively separating formaldehyde from the reaction mixtures of the condensation. In connection with the present invention, the references cited below are of interest.
DD-A-273 434 describes a process for separating excess formaldehyde from reaction solutions, in which formaldehyde is reacted with its higher alkanals by the classical Cannizzaro process. A crude mixture containing the polyhydric alcohol and the formaldehyde used in excess is obtained. Formaldehyde is separated by addition of methanol, water and/or acetonitrile and subsequent azeotropic distillation of the formaldehyde as a complex mixture with the added solvents and further components. The polyhydric alcohol remains in the bottom.
Japanese Patent JP 10287606 (cited according to CA 1998/129: 275637) describes a process for removing formaldehyde from reaction mixtures which originate from the reaction of this aldehyde with higher aldehydes and contain the corresponding dimethylolalkanal as a product. The removal is effected by distillation in a thin-film evaporator. Before the distillation, water is added to the mixture, in amounts of  greater than 4 parts by weight. As a result of the distillation, a residual formaldehyde content of 5% is reached, at pressures of from 0.5 to 1 bar.
U.S. Pat. No. 4,036,888 also describes the addition of water to reaction mixtures of the reaction of formaldehyde with higher aldehydes and subsequent distillation. In this case, isobutyraldehyde is used and the product is hydroxypivalaldehyde. By adding water, the isobutyraldehyde used in excess is effectively removed here. For this reason, the problem of removing formaldehyde does not exist here since it is present after the reaction only in very small amounts.
The same also applies to U.S. Pat. No. 5,235,118, which relates to the reaction of 2-ethylhexanal with formaldehyde to give 2-ethyl-2-(hydroxymethyl)hexanal. Here too, formaldehyde is used in substoichiometric amounts relative to the 2-ethylhexanal, and only a small residue of formaldehyde is present in the product solution. The addition of water with subsequent distillation serves for effective removal of unreacted 2-ethylhexanal.
It is an object of the present invention to provide a process which permits effective removal of formaldehyde from the methylolalkanals obtained after reaction of formaldehyde with higher aldehydes. The separation should be so effective that the subsequent hydrogenation of the methylolalkanal is economical.
We have found that this object is achieved by a process for the removal, by distillation, of formaldehyde from reaction solutions containing a methylolated alkanal which was obtained from the reaction of formaldehyde with an alkanal which has at least one acidic hydrogen atom ax to the carbonyl function or from the reaction of a 2-alkylacrolein or acrolein with water and formaldehyde, this reaction having been carried out in the presence of catalytic amounts of an organic amine, wherein a suitable amount of water is added before and/or during distillation.
We have found that this object is furthermore achieved by a process for the preparation of polyols, wherein a methylolated alkanal which was freed from formaldehyde according to the process described above is subjected to a hydrogenation known per se.
It was found that effective separation can be achieved by adding water before and/or during the removal of formaldehyde by distillation from the methylolalkanal obtained after the condensation of formaldehyde with a higher aldehyde. Furthermore, the yield in a subsequent hydrogenation can be increased by adding water.
In general, the methylolalkanals will be di- or trimethylolalkanals. The methylolalkanals are generally prepared by condensation of the abovementioned starting compounds in the presence of catalytic amounts of an organic amine. In the present process, it is possible in general to use virtually all formaldehyde-containing discharges from an aldol condensation which are prepared by said condensation reaction. The process can be particularly advantageously used in the case of discharges from an aldol condensation which were obtained according to WO 98/28253. The content of this application is hereby incorporated by reference in the present application.
The removal of the formaldehyde from the aldol condensation mixtures is effected in a suitable distillation apparatus. Such apparatuses are known to those skilled in the art. It is preferable to use a distillation column.
Suitable distillation columns have from 3 to 100 theoretical plates. The best results were obtained with from 5 to 60 theoretical plates. These theoretical plates can be achieved by the usual measures. Trays, stacked packings and dumped packings may be mentioned here as non-restricting examples. These theoretical plates are preferably designed in such a way that residence times of from 1 to 300, particularly preferably from 10 to 180, minutes are achieved.
The column is operated in the distillation at from 50 to 160xc2x0 C. The chosen pressure is from 20 mbar to 5 bar, preferably from 1 to 3 bar, in particular from 1.5 to 3 bar. The column is adjusted in such a way that a reflux ratio of from 100 to 0, preferably from 3 to 0, results. The reaction mixture to be separated is preferably fed in in the upper half of the column or at the top of the column.
To permit effective removal of the formaldehyde, water is fed in at a suitable point of the column. Water can be added in the form of liquid water or, preferably, in the form of steam. The feed can be effected at various points in the distillation column or the bottom, a feed with the feed stream or below it being preferred. A feed into the bottom of the column or into the evaporator is particularly preferred. The water is metered in such a way that its amount is from 0.1 to 10 parts of the feed stream. Owing to the temperature sensitivity of the product mixture, it is preferred in connection with the present invention to operate the column with an evaporator which has a low wall temperature and a small liquid content. It has proven particularly advantageous to employ a falling-film evaporator.
In a preferred embodiment of the present invention, the bottom of the column and the bottom of the evaporator are designed in such a way that the residence time of the bottom discharge, i.e. of the methylolated alkanal, from which formaldehyde has been removed, in water is from 1 to 300 minutes. Particularly preferred residence times are from 5 to 120 minutes. In the case of these residence times chosen in this manner, an optimum is achieved with regard to removal of formaldehyde and avoidance of the formation of undesirable byproducts.
The product stream which is obtained after the purification and consists of a mixture of the methylolated alkanal with water is generally removed from the bottom of the column or, if the distillation apparatus has a divided bottom, from the evaporator circulation stream obtained downstream of the evaporator or from the bottom of the evaporator.
Owing to the broad condensation curve of the vapor emerging at the top of the column, it is advantageous to use a condenser with liquid recycling. The direct condensation in a quench with cooled liquid circulation is particularly advantageous.
The novel process can be used for removing formaldehyde from all mixtures which result from its reaction with alkanals or its reaction with water and acrolein or 2-alkylacrolein. Examples of aldehydes are aliphatic aldehydes of 2 to 24 carbon atoms which may have a linear or branched chain or may contain alicyclic groups. All that is required is that at least one acidic hydrogen atom is present a to the carbonyl function. This also applies to aralkylaldehydes, which may be used as starting material. In general, those aldehydes of 8 to 24, preferably 8 to 12, carbon atoms are used. Examples of suitable aldehydes are 3-ethyl-, 3-n-propyl-, 3-isopropyl-, 3-n-butyl-, 3-isobutyl-, 3-sec-butyl- and 3-tert-butylbutanal and corresponding n-pentanals, n-hexanals and n-hepthnals; 4ethyl-, 4-n-propyl-, 4-isopropyl-, 4-n-butyl-, 4-isobutyl-, 4-sec-butyl- and 4-tert-butylpentanals, -n-hexanals and -n-heptanals; 5-ethyl-, 5-n-propyl-, 5-isopropyl-, 5-n-butyl-, 5-isobutyl-, 5-sec-butyl- and 5-tert-butyl-n-hexanals and -n-heptanals; 3-methylhexanal, 3-methylheptanal; 4-methylpentanal, 4-methylheptanal, 5-methylhexanal, 5-methylheptanal; 3,3,5-trimethyl-n-pentyl-, 3,3-diethylpentyl-, 4,4-diethylpentyl-, 3,3-dimethyl-n-butyl-, 3,3-dimethyl-n-pentyl-, 5,5-dimethyl-heptyl-, 3,3-dimethylheptyl-, 3,3,4-trimethylpentyl-, 3,4-dimethylheptyl-, 3,5-dimethylheptyl-, 4,4-dimethylheptyl-, 3,3-diethylhexyl-, 4,4-dimethylhexyl-, 4,5-dimethylhexyl-, 3,4-dimethylhexyl-, 3,5-dimethythexyl-, 3,3-dimethylhexyl-, 3,4-diethylhexyl-, 3dimethyl4-ethylpentyl-, 3-methyl-4-ethylhexyl-, 3,3,4-trimethylpentyl-, 3,4,4-trinethylpentyl-, 3,3,4-trimethylhexyl-, 3,4,4-trimethylhexyl- and 3,3,4,4-tetramethylpentylaldehyde; in particular C2- to C12-n-alkanals.
Examples of alkylacroleins are methacrolein, 2-ethylacrolein, 2-propylacrolein and 2-hydroxymethylacrolein.
Particularly preferably used starting compounds are acetaldehyde for the preparation of pentaerytritol, propionaldehyde for the preparation of trimethylolethane, n-butyraldehyde for the preparation of TMP, n-pentanal for the preparation of trimethylolbutane and isobutyraldehyde for the preparation of neopentylglycol.
The purified product stream is then subjected to a hydrogenation in order to convert the alkanal function into an alcohol function and thus to prepare polyalcohols, in general triols or tetrahydric alcohols or diols. It was found that the product mixtures which result from the process according to the present invention and have a higher water content than the product mixtures described to date give particularly advantageous results in the hydrogenation. It was surprisingly found that the higher water content is not disadvantageous but the opposite. For example, it was found that, as a result of the increased dilution, the subsequent hydrogenation is substantially more selective with respect to the alkanal. Furthermore, the increased dilution also has an advantageous effect on the life of the catalyst, which is deactivated substantially more slowly.
The present invention furthermore relates to a process for the hydrogenation of methylolalkanals which were obtained by the process according to the present invention and have a higher water content.
Hydrogenation is carried out under the conditions known in the literature and using the known materials. Copper-containing catalysts are preferably used. Further preferred catalysts are described in the applications DE-A-24 45 303, DE-A-198 09 418, DE-A-197 30 939 and WO 95/32171. The hydrogenations are carried out at from 5 to 250, preferably from 10 to 200, particularly preferably from 20 to 150, bar.
The examples which follow illustrate the invention. In these examples, the aldol condensation mixture used had been prepared as follows:
An apparatus consisting of two heatable stirred kettles connected to one another by overflow pipes and having a total capacity of 72 l was continuously fed with fresh, aqueous formaldehyde solution (4300 g/h) in the form of a 40% strength aqueous solution or n-butyraldehyde (1800 g/h) and with fresh trimethylamine as catalyst (130 g/h) in the form of a 45% strength aqueous solution. The reactors were heated to 40xc2x0 C. It is possible to use a formaldehyde mixture having a low methanol content, as described in the German application with the tide xe2x80x9cVerfahren zur Herstellung von Polyalkoholen mit Methanol-armem Formaldehydxe2x80x9d [Process for the preparation of polyalcohols using formaldehyde having a low methanol content], file reference 199 63 438.6 (applicant: BASF AG).
The discharge was passed directly into the upper part of a falling-film evaporator with attached column (11 bar heating steam) and separated there by distillation under atmospheric pressure into a low-boiling top product, essentially containing n-butyraldehyde, ethylacrolein, formaldehyde, water and trimethylamine, and a high-boiling bottom product.
The top product was condensed continuously and was recycled into the reactors described above.
The high-boiling bottom product from the evaporator (about 33.5 kg/h) was continuously mixed with fresh trimethylamine catalyst (50 g/h, in the form of the 45% strength aqueous solution) and fed into a heatable tubular reactor provided with packings and having an empty volume of 12 l. The reactor was heated to 40xc2x0 C.
The discharge of the downstream reactor was fed continuously into the upper part of a further distillation apparatus, for the removal of formaldehyde (11 bar heating steam) and separated there by distillation into a low-boiling top product, essentially containing ethylacrolein, formaldehyde, water and trimethylamine, and a high-boiling bottom product. The low-boiling top product (27 kg/h) was condensed continuously and was recycled into the first stirred kettle, whereas the high-boiling bottom product was collected.
The bottom product thus obtained contained essentially dimethylolbutyraldehyde, formaldehyde and traces of monomethylolbutyraldehyde in addition to water.